Optical pulse generation for an extreme ultraviolet light source

ABSTRACT

An optical pulse for an extreme ultraviolet (EUV) light source may be formed by illuminating a semiconductor material of a modulation system with a first light beam having a first wavelength; applying a voltage to the semiconductor material for a time duration, the applied voltage being sufficient to modify an index of refraction of the semiconductor material such that a polarization state of a light beam having a second wavelength passing through the semiconductor material is modified to pass through at least one polarization-based optical element of the modulation system; and forming an optical pulse by passing a second light beam having the second wavelength through the semiconductor material during the time duration.

TECHNICAL FIELD

This disclosure relates to optical pulse generation for an extremeultraviolet light source.

BACKGROUND

Extreme ultraviolet (“EUV”) light, for example, electromagneticradiation having wavelengths of around 50 nm or less (also sometimesreferred to as soft x-rays), and including light at a wavelength ofabout 13 nm, may be used in photolithography processes to produceextremely small features in substrates, for example, silicon wafers.

Methods to produce EUV light include, but are not necessarily limitedto, converting a material that has an element, for example, xenon,lithium, or tin, with an emission line in the EUV range in a plasmastate. In one such method, often termed laser produced plasma (“LPP”),the required plasma may be produced by irradiating a target material,for example, in the form of a droplet, plate, tape, stream, or clusterof material, with an amplified light beam that may be referred to as adrive laser. For this process, the plasma is typically produced in asealed vessel, for example, a vacuum chamber, and monitored usingvarious types of metrology equipment.

SUMMARY

In one general aspect, a method of forming an optical pulse for anextreme ultraviolet (EUV) light source includes illuminating asemiconductor material of a modulation system with a first light beamhaving a first wavelength; applying a voltage to the semiconductormaterial for a time duration, the applied voltage being sufficient tomodify an index of refraction of the semiconductor material such that apolarization state of a light beam having a second wavelength passingthrough the semiconductor material is modified to pass through at leastone polarization-based optical element of the modulation system; andforming an optical pulse by passing a second light beam having thesecond wavelength through the semiconductor material during the timeduration. The formed optical pulse includes a first portion and a secondportion, the first portion and the second portion being temporallycontiguous, the first portion occurring before the second portion, andilluminating the semiconductor material of the modulation system withthe first light beam modifies one or more characteristics of the firstportion of the formed optical pulse.

Implementations may include one or more of the following features. Theone or more characteristics of the first portion of the optical pulsemay include one or more of an average intensity, a maximum intensity,and a temporal duration. The formed pulse may be allowed to propagatetoward a target region, and the first portion of the formed pulse mayhave a maximum intensity that is less than a maximum intensity of thesecond portion of the formed pulse. The maximum intensity of the secondportion may be sufficient to convert target material in a target at thetarget region to a plasma that emits EUV light.

The second wavelength may be at least three times greater than the firstwavelength.

The semiconductor material may be associated with a spectraltransmission characteristic, the spectral transmission characteristicincluding a transmission region and an absorption edge wavelength, theabsorption edge wavelength being the lowest wavelength on thetransmission region, the first wavelength is between the absorption edgewavelength and the second wavelength, and the second wavelength may be awavelength in the transmission region. The second wavelength is no morethan 3.5 times greater than the first wavelength.

The semiconductor material may be associated with a band gap energy, theband gap energy being an energy difference between a valence band of thesemiconductor material and a conduction band of the semiconductormaterial, and the photon energy of the first wavelength may be less thanthe band gap energy. The semiconductor material may include defects, thedefects creating deep-level traps with energy levels between the valenceband and the conduction band, and the photon energy of the firstwavelength may be equal to or greater than an energy difference betweenat least one energy level of a deep-level trap and the conduction bandor between at least one energy level of a deep-level trap and thevalence band.

The second wavelength may include 10.6 μm, and the semiconductormaterial may be one of cadmium zinc telluride (CdZnTe), cadmiumtelluride (CdTe), zinc telluride (ZnTe), and gallium arsenide (GaAs).The first wavelength may be a wavelength between 0.75 microns (μm) and3.5 μm, and the second wavelength may include a wavelength between 9 μmand 11 μm.

The first light beam and the second light beam may follow the samespatial path through the semiconductor material. The first light beamand the second light beam may be in the semiconductor material at thesame time.

A property of the first light beam may be adjusted to adjust one or moreof the characteristics of the first portion of the optical pulse.Adjusting the property of the first light beam may include increasing anintensity of the first light beam to reduce a maximum or averageintensity of the first portion of the optical pulse.

In another general aspect, a system for an extreme ultraviolet (EUV)light source includes a modulation system including a semiconductormaterial, the semiconductor material including one or more types ofdefects, an index of refraction of the semiconductor material varying inresponse to application of voltage; a first light source configured toproduce a first light beam having a first wavelength, where illuminatingthe semiconductor material with light having the first wavelengthincreases a leakage current of the semiconductor material; and a controlsystem coupled to the modulation system, the control system configuredto cause a voltage to be applied to the semiconductor material while asecond light beam having a second wavelength propagates in thesemiconductor material to form an optical pulse from the second lightbeam, the optical pulse configured to convert at least some targetmaterial to plasma that emits EUV light.

Implementations may include one or more of the following features. Thepulse may include a first portion and a second portion, the firstportion and the second portion being temporally contiguous, and thefirst portion occurring before the second portion.

The modulation system also may include at least one polarization-basedoptical element. The semiconductor material may be a crystal. The secondlight beam may be a continuous light beam. The types of defects mayinclude one or more of precipitates, inclusions, twins, and slip-planes.

The second light source may include a pulsed light source, the controlsystem may be coupled to the modulation system and the second lightsource, and the control system may be configured to control the secondlight source to emit a pulse of light. The control system also may beconfigured to control the second light source to direct a pulse of thesecond light beam toward the semiconductor material while the voltage isapplied to the semiconductor material and while the first light beam isdirected toward the semiconductor material. At least one pulse of thefirst light beam and the second light beam may be in the semiconductormaterial at the same time.

The control system also may be configured to control the first lightsource to thereby control one or more characteristics of the firstportion of the optical pulse. The one or more characteristics of thefirst portion may include one or more of an average intensity, a maximumintensity, and a temporal duration. The control system may be configuredto control an intensity of the first portion of the pulse by controllingthe first light source.

The system for the EUV light source also may include an optical beamdelivery system between the first light source and the semiconductormaterial, the optical beam delivery system being configured to directthe first light beam to a particular location on the semiconductormaterial.

In another general aspect, a method of modifying an acoustic effect inan electro-optic modulator includes applying a voltage to asemiconductor of the electro-optic modulator during a first time period,the application of the voltage generating an acoustic effect in thesemiconductor, the acoustic effect including an oscillating acousticwave; and illuminating the semiconductor with a seed light beam, theseed light beam having a wavelength that has a photon energy that isless than a band gap energy of the semiconductor to thereby modify oneor more of an amplitude and a frequency of the acoustic wave.

Implementations may include one or more of the following features. Acontinuous wave light beam may be directed toward semiconductor of theelectro-optic modulator, where a first amount of the continuous wavelight beam passes through the electro-optic modulator with a firstpolarization state during the first time period while the voltage isapplied to the semiconductor, a second amount of the continuous wavelight beam passes through the electro-optic modulator at a time not inthe first time period when the voltage is not applied to thesemiconductor and the acoustic effect is present in the semiconductor,and illuminating the semiconductor with the seed light beam changes thesecond amount of the light beam that passes through the electro-opticmodulator.

A pulse of a pulsed light beam may be directed toward theelectro-optical modulator, a first amount of light of the pulse passingthrough the semiconductor during the first time period when the voltageis applied to the semiconductor, and a second amount of light of thepulse passing through the semiconductor during a time that is not in thefirst time period and when the voltage is not applied to thesemiconductor, where illuminating the semiconductor with the seed lightbeam changes the second amount of light.

Implementations of any of the techniques described above may include anEUV light source, a system, a method, a process, a device, or anapparatus. The details of one or more implementations are set forth inthe accompanying drawings and the description below. Other features willbe apparent from the description and drawings, and from the claims.

DRAWING DESCRIPTION

FIG. 1 is a block diagram of an example of an extreme ultraviolet (EUV)lithography system that includes an EUV light source.

FIG. 2A is a block diagram of a modulation module used in the EUV lightsource of FIG. 1.

FIG. 2B is an illustration of an optical pulse.

FIG. 3A is a plot of optical leakage of the modulation module of FIG. 2Aand two pulses formed by the modulator of FIG. 2A.

FIG. 3B is an example of a timing diagram to apply to a modulationmodule to produce pulses shown in FIG. 3A.

FIG. 4A is an example of an energy diagram for a semiconductor of themodulation module of FIG. 2A.

FIG. 4B is an example of a spectral transmission characteristic for asemiconductor of the modulation module of FIG. 2A.

FIG. 5A is a plot of two optical pulses that may be produced by themodulation module of FIG. 2A.

FIG. 5B is an example of a timing diagram to apply to a modulationmodule to produce pulses shown in FIG. 5A.

FIG. 6 is a flow chart of an example of a process of forming an opticalpulse.

FIGS. 7-10 are examples of experimental results.

FIGS. 11 and 12 are perspective illustrations of pulse generatingsystems that may be used in the EUV light source of FIG. 1.

FIGS. 13A and 13B are block diagrams of an example of a EUV lightsource.

DETAILED DESCRIPTION

Techniques for forming an optical pulse in an extreme ultraviolet (EUV)light source are discussed. An electro-optic modulator used to form theoptical pulse is illuminated with a seed light beam to liberate trappedelectrical charges in a semiconductor of the electro-optic modulator.The charges become trapped due to defects that are present in thesemiconductor, and the trapped charges increase the optical leakage ofthe modulator. The optical leakage contributes additional or spuriouslight to the pulses formed by the modulator. Some of this spurious lightforms a “pedestal” at the beginning of the formed pulse. By liberatingthe trapped charges, the optical leakage is reduced and/or controlled,and the pedestal also may be reduced and/or controlled.

Referring to FIG. 1, a block diagram of a system 100 is shown. Thesystem 100 is an example of an EUV lithography system. The system 100includes an EUV light source 101, which provides EUV light 196 to alithography apparatus 195. The lithography apparatus 195 exposes a wafer(for example, a silicon wafer) with the EUV light 196 to form electronicfeatures on the wafer. The EUV light 196 is emitted from a plasma thatis formed by irradiating target material in a target 118 with an opticalpulse 107. The target material is any material (for example, tin) thatemits EUV light in a plasma state.

The EUV light source 101 includes a pulse generating system 104, whichproduces the pulse 107. The pulse generating system 104 includes a lightsource 105, which may be, for example, a pulsed (for example, aQ-switched) or continuous-wave carbon dioxide (CO₂) laser. The lightsource 105 produces a light beam 106, which may be a train of pulses oflight or a continuous light beam. The light source 105 emits the lightbeam 106 toward a modulation module 120 that includes a semiconductor122.

The modulation module 120 is an electro-optic modulator that modulatesan incident light beam based on the electro-optic effect. Theelectro-optic effect describes the change in the refractive index of amaterial (the semiconductor 122) resulting from the application of adirect-current (DC) or low-frequency electric field. The modulationmodule 120 is controlled by a control system 175 to form the pulse 107from the light beam 106 or from a pulse of light of the light beam 106.The pulse 107 propagates on a path 111 toward a vacuum vessel 180, whichreceives the target 118. The pulse 107 and the target 118 interact at atarget region 115 in the vacuum vessel 180, and the interaction convertsat least some of the target material in the target 118 into plasma thatemits the EUV light 196.

The EUV light source 101 also includes a seed light source 110. The seedlight source 110 emits a seed light beam 114, which illuminates thesemiconductor 122. The semiconductor 122 includes defects, which maytrap electrical charges. The seed light beam 114 has a wavelengthassociated with a photon energy that is sufficient to excite the trappedcharges and may migrate the trapped charges to a conduction band of thesemiconductor 122, increasing the conductivity of the semiconductor 122.As discussed in greater detail below with respect to FIGS. 2-12,removing or reducing the trapped charges improves the ability of themodulation module 120 to form the pulse 107.

The control system 175 exchanges data and/or information with the pulsegenerating system 104 and/or any of the components of the pulsegenerating system 104 via a communications interface 176. For example,in some implementations, the control system 175 may provide triggersignals to operate the modulation module 120 and/or the light source105. The control system 175 includes an electronic processor 177, anelectronic storage 178, and an input/output (I/O) interface 179. Theelectronic processor 177 includes one or more processors suitable forthe execution of a computer program such as a general or special purposemicroprocessor, and any one or more processors of any kind of digitalcomputer. Generally, an electronic processor receives instructions anddata from a read-only memory, a random access memory, or both. Theelectronic processor 177 may be any type of electronic processor.

The electronic storage 178 may be volatile memory, such as RAM, ornon-volatile memory. In some implementations, and the electronic storage178 includes non-volatile and volatile portions or components. Theelectronic storage 178 may store data and information that is used inthe operation of the control system 175 and/or components of the controlsystem 175.

The electronic storage 178 also may store instructions, perhaps as acomputer program, that, when executed, cause the processor 177 tocommunicate with components in the control system 175, the modulationmodule 120, and/or the light source 105. For example, in implementationsin which the source 105 is a pulsed source, the instructions may beinstructions that cause the electronic processor 177 to generate asignal that results in the light source 105 emitting an optical pulse.

The I/O interface 179 is any kind of electronic interface that allowsthe control system 175 to receive and/or provide data and signals withan operator, the modulation module 120, and/or the light source 105,and/or an automated process running on another electronic device. Forexample, the I/O interface 179 may include one or more of a visualdisplay, a keyboard, and a communications interface.

FIG. 2A is a block diagram of the modulation module 120. The modulationmodule 120 includes the semiconductor 122, which is positioned betweenelectrodes 123 a, 123 b. The electrodes 123 a, 123 b are controllable toform an electric field between the electrode 123 a and 123 b. Forexample, the control system 175 may cause the electrode 123 a to be heldat a greater voltage than the electrode 123 b, thus creating an electricfield or a potential difference (V) across the semiconductor 122.

The modulation module 120 also includes one or more polarization-basedoptical elements 124. In the example of FIG. 2A, only onepolarization-based optical element 124 is shown. However, in otherimplementations, additional polarization-based optical elements 124 maybe included. For example, a second polarization-based optical element124 may be on a side of the modulation module 120 that receives thelight beam 106.

The polarization-based optical element 124 is any optical element thatinteracts with light based on the polarization state of the light. Forexample, polarization-based optical element 124 may be a linearpolarizer that transmits horizontally polarized light and blocksvertically polarized light, or vice versa. The polarization-basedoptical element 124 may be a polarizing beam splitter that transmitshorizontally polarized light and reflects vertically polarized light.The polarization-based optical element 124 may be an optical elementthat absorbs all light except for light having a particular polarizationstate. In some implementations, the polarization-based optical element124 may include a quarter-wave plate. At least one polarization-basedoptical element 124 is positioned to receive light that passes throughthe semiconductor 122 and to direct light of a certain polarizationstate onto the beam path 111.

The semiconductor 122 may be any material that transmits one of morewavelengths of the light beam 106. The semiconductor 122 also exhibitsanisotropy that can be modified by application of a controllableexternal force (such as the potential difference (V)). The control overanisotropy enables control over indices of refraction for differentpolarization components of light propagating through the semiconductor122. For implementations in which the light beam 106 includes light of awavelength of 10.6 microns (μm), the semiconductor 122 may be, forexample, cadmium zinc telluride (CdZnTe or CZT), cadmium telluride(CdTe), zinc telluride (ZnTe), and/or gallium arsenide (GaAs). Othermaterials may be used at other wavelengths. For example, thesemiconductor 122 may be monopotassium phosphate (KDP), ammoniumdihydrogen phosphate (ADP), quartz, cuprous chloride (CuCl), zincsulphide (ZnS), zinc selenide (ZnSe), lithium niobate (LiNbO₃), galliumphosphide (GaP), lithium tantalate (LiTaO₃), or barium titanate(BaTiO₃). Other materials that transmit one or more wavelengths of thelight beam 106 and exhibit birefringence in response to application ofan external force also may be used.

The polarization state of the light that passes through thesemiconductor 122 may be controlled by controlling the potentialdifference (V) between the electrodes 123 a, 123 b. Under idealoperation, the modulation module 120 only emits light when the potentialdifference V applied to the semiconductor 122 causes the polarizationstate of the light passing through the semiconductor 122 to match thepolarization-based optical element 124. For example, if thepolarization-based optical element 124 is a linear polarizer positionedto transmit horizontally polarized light onto the beam path 111, and thelight beam 106 is vertically polarized when initially incident on thesemiconductor 122, the pulse 107 is only formed when the potentialdifference V applied to the semiconductor 122 changes the polarizationstate of the light beam 106 such that the light beam 106 is horizontallypolarized.

However, in actual operation, the modulation module 120 also emitsspurious light (optical leakage). This optical leakage is presentthroughout the operation of the modulation module 120, and causes anadditional amount of light to pass through the modulation module 120,including at times when no light should pass through the modulationmodule 120. For example, the additional light causes an additional orextra amount at the beginning of the pulse 107, and this additionallight forms the pedestal portion.

Referring also to FIG. 2B, an illustration of the pulse 107 with apedestal 125 is shown. FIG. 2B shows the intensity of the pulse 107 as afunction of time. The pedestal 125 occurs during a window labeled as121, and the pedestal 125 occurs earlier in time than the rest of thepulse 107. The portions of the pulse 107 that are not the pedestal 125are referred to as the main portion 168. The pedestal 125 and the mainportion 168 are both part of the pulse 107, and the pedestal 125 istemporally connected to the main portion 168.

The average and maximum intensity and optical energy of the pedestal 125is less than the average and maximum intensity and optical energy ofmain portion 168. The main portion 168 has an intensity or energysufficient to convert at least some of the target material in the target118 into plasma that emits EUV light. The pedestal 125 does not have asmuch energy, and may not have sufficient energy to convert the targetmaterial into plasma. However, the light in the pedestal 125 may reflectoff of the target 118, evaporate material from the surface of the target118, and/or break off parts of the target 118.

The pedestal 125 occurs before the main portion 168 and reaches thetarget 118 before the main portion 168. The pedestal 125 may interferewith the plasma formation by altering the target 118 before the mainportion 168 reaches the target 118 and/or cause undesirable reflectionsthat propagate back on the path 111. As such, it is desirable to controlthe amount of light in the pedestal 125. Illuminating the semiconductor122 with the seed light beam 114 also allows one or more characteristicsof the pedestal 125 to be changed, controlled, reduced, or eliminated.For example, the average amount of light and/or the maximum amount oflight in the pedestal 125 may be reduced.

FIG. 3A shows plots of optical leakage of an optical modulator as afunction of time without the benefit of irradiation by the seed lightbeam 114. The example shown in FIG. 3A is discussed with respect to themodulation module 120 and the semiconductor 122. Pulses 307_1 and 307_2(FIG. 3A) are produced by applying a potential difference to thesemiconductor 122 of the modulation module 120 (FIG. 3B). The opticalleakage is the percentage of light incident on the modulation module 120that passes through the modulation module 120 even when the potentialdifference is not applied to the semiconductor 122.

In the example of FIG. 3A, a continuous wave light beam is incident onthe semiconductor 122. Referring also to FIG. 3B, a timing diagram 300Bis shown. The timing diagram 300B is an example of voltages applied tothe semiconductor 122 during the time period illustrated in FIG. 3A.When the potential difference V is applied to the semiconductor 122, andthe polarization of the light emerging from the semiconductor 122 ismatched to the polarization-based optical element 124, a relatively highpercentage of incident light passes through the modulation module 120.Pulses 307_1 and 307_2 are formed from light that passes through themodulation module 120 while the potential difference V is applied to thesemiconductor 122.

The timing diagram 300B shows four periods 333-336. The period 333includes time (t=0) to time (t=t2). The potential difference V is notapplied to the semiconductor 122 during the period 333. The period 334begins at time (t=t2) and ends at time (t=t3). The potential differenceV is applied to the semiconductor 122 during the period 334 to form thepulse 307_1. The period 335 begins at time (t=t3) and ends at time(t=t5). The potential difference V is not applied to the semiconductor122 during the period 335. The potential difference V is again appliedto the semiconductor 122 during the period 336 (which begins at t=t5 andends at t=t6) to form the pulse 307_2.

The temporal duration of the periods 333-336 depends on thecharacteristics of the modulation module 120 and the operatingparameters of the EUV light source 101. For example, the rise time ofthe modulation module 120 (the time for the semiconductor 122 to respondto an applied potential difference) may determine the minimum durationof the periods 334 and 336. The desired temporal duration of the pulses307_1 and 307_2 also may determine the duration of the periods 334 and336. In some implementations, the periods 334 and 336 may be, forexample, 50-300 nanoseconds (ns), 50-1000 ns, or 50-100 ns. The period335 may be, for example, 20,000 ns. The example of FIG. 3A shows twopulses 307_1 and 307_2. However, additional pulses may be formed byapplying the potential difference V again at a later time. For example,pulses may be produced at 50 kHz by applying the potential difference Vto the semiconductor 122 every 20,000 ns.

In addition to the pulses 307_1 and 307_2, spurious light (for example,optical leakage) emerges from the modulation module 120 at other timesdue to defects in the semiconductor 122 and other effects. The opticalleakage has several components, all of which are types of opticalleakage: static leakage 328, acoustic leakage 329, and dynamic leakageoffset (DLO) 330. The static optical leakage 328 is present regardlessof whether the potential difference V is applied or not. The acousticleakage 329 and the DLO 330 arise due to the application of thepotential difference V. Together, the acoustic leakage 329 and the DLO330 may be considered dynamic optical leakage.

The static optical leakage 328 is the percentage of incident light thatpasses through the modulation module 120 regardless of whether thepotential difference V is applied to the semiconductor 122. In theexample of FIG. 3B, a trace 327 shows the static optical leakage 328 asa function of time.

The dynamic optical leakage also includes the acoustic leakage 329.Applying the potential difference V affects the piezo-electricproperties of the semiconductor 122 and generates acoustic waves thatpropagate in the semiconductor 122. The acoustic waves persist after thepotential difference V is removed. The presence of the acoustic waves inthe semiconductor 122 changes the index of refraction of thesemiconductor 122 and thus may result in unintentional transmission oflight through the modulation module 120 during the time period 335. Theportion or percentage of incident light that is emitted as acousticleakage 329 oscillates over time between a minimum acoustic leakage 326and a peak acoustic leakage 329 p. The acoustic leakage 329 is alsodamped over the time period 335 (the amount of acoustic leakagedecreases from the peak 329 p with each subsequent oscillation thatoccurs in the period 335). The characteristics of the dampening dependon the properties of the semiconductor, and the oscillations may notfully dampen before the next application of the potential difference V.For example, the oscillations may not fully dampen until about 100 μsafter the application of the potential difference V, and the time period335 may be, for example, about 20 μs. Thus, the oscillations may stillbe present in the semiconductor 122 when the potential difference V isapplied again.

The dynamic optical leakage also includes the DLO 330. The DLO 330 isthought to be caused primarily by the presence of charges (for example,electrons) trapped in defects in the semiconductor 122. The trappedcharges create an electric field within the semiconductor 122 andprevent the applied potential difference V from being uniform throughoutthe semiconductor 122.

Thus, although the potential difference V is applied to thesemiconductor during the time periods 334 and 336 to form the respectivepulses 307_1 and 307_2, the presence of the optical leakage results inthe pulses 307_1 and 307_2 including light that arises from spurioussources. For example, the pulses 307_1 and 307_2 include a respectivepedestal portion 325 a, 325 b. The pedestal portions 325 a, 325 b occurbefore the remaining portions of the respective pulse 307_1, 307_2. Inthe example of FIG. 3A, the pedestal portions 325 a, 325 b are shown asa percentage of transmission of the average total optical leakage withina 400 nanosecond (ns) window. The windows are labeled as 321 a and 321b. The window 321 a is from time=t1 to time=t2, and the window 321 b isfrom time=t4 to time=t5. The pedestal portions 325 a, 325 b occur whenthe potential difference V is not applied to the semiconductor 122.

In the example of FIG. 3A, the DLO 330 is the largest component of thetotal optical leakage, and a large portion of the pedestal portion 325a, 325 b may be removed by reducing or eliminating the DLO 330. Forexample, the DLO 330 may account for between 33% and 66% of the totaloptical leakage. In one example, the total optical leakage is 0.3%, thestatic leakage 328 is 0.07%, the acoustic leakage 329 is 0.07%, and theDLO 330 accounts for 0.16%. However, other examples may have differentparameters. For example, CZT (which may be used as the semiconductor122) may have a DLO of 2%. In another example, optical leakage in amodulation module that uses CdTe as the semiconductor 122 may have arelatively low DLO component and a more dominant acoustic component. Inthese implementations, the reduction of the acoustic component of theoptical leakage has a larger impact on the reduction or elimination ofthe pedestal portion. As discussed with respect to FIGS. 4A and 4B, theDLO 330 and the acoustic leakage 329 is reduced by illuminating thesemiconductor 122 with the seed light beam 114.

Referring to FIG. 4A, an illustration of an electronic band structure440 (energy versus carrier momentum) of the semiconductor 122 is shown.The electronic band structure 440 describes the range of energies thatan electron in the semiconductor 122 may have. The semiconductor 122 hasa band gap energy (Eg) 441, which is the energy difference (in electronvolts or eV) between a conduction band 442 and a valence band 443. Anelectron (for example, the electron 445) may migrate from the valenceband 443 to the conduction band 442 if the electron gains enough energyto move to the conduction band 442. The electron may gain energy byabsorbing either a phonon (heat) or a photon (light). Electrons in theconduction band 442 flow as electrical current.

The semiconductor 122 includes defects, such as, for example,precipitates, inclusions, twins, and/or slip-planes. The defects act assites where charges (for example, an electron 446) become trapped. Inthe example of FIG. 4A, a deep-level trap 444 is formed from a defect.The deep-level trap 444 is associated with a trap gap energy 447, whichis the energy difference between the deep-level trap 444 and theconduction band 442. The trap gap energy 447 is less than the band gapenergy 441.

Referring also to FIG. 4B, a spectral transmission characteristic 450for the semiconductor 122 is shown. The spectral transmissioncharacteristic 450 represents the percentage transmission as a functionof wavelength of incident light. In the spectral transmissioncharacteristic 450, the lowest wavelength (or the shortest wavelength)is on the leftmost side of the horizontal axis, with the wavelengthincreasing (or becoming longer) as the x axis increases to the right.

The spectral transmission characteristic 450 includes a transmissionregion 452, an electronic absorption region 451, and an absorption edge453. The absorption edge 453 is the interface or boundary between thetransmission region 452 and the electronic absorption region 451. Thewavelength of the absorption edge 453 has a photon energy thatcorresponds to the band gap energy 441. A photon having a wavelength (A)has a photon energy (E) provided by Equation 1:

$\begin{matrix}{{E = \frac{hc}{\lambda}},} & {{Equation}\mspace{14mu} 1}\end{matrix}$where h is the Planck constant and c is the speed of light.

The semiconductor 122 may be CdZnTe. In this example, the transmissionregion 452 is between about 0.8 microns (μm) and 20 (μm), and theabsorption edge is about 730 nanometers (nm). For light having awavelength in the transmission region, about 60% of incident light istransmitted. In another example, the semiconductor 122 is CdTe. In thisexample, the transmission region 452 is between about 0.9 μm to 20 μm,and the absorption edge is about 861 nm. For light having a wavelengthin the transmission region, about 65% of incident light is transmitted.Light having a wavelength in the electronic absorption region 451 has aphoton energy greater than the band gap energy 441. Such photons arelikely to be absorbed by the semiconductor 122.

The light beam 106, which is used to form the pulse 107, has awavelength in the transmission region 452 and is thus transmittedthrough the semiconductor 122. The light beam 114, which is used toilluminate the semiconductor 122, may have a broad spectrum and mayinclude many wavelengths (and thus many different photon energies),including wavelengths that have a photon energy that is less than theband gap energy 441. Light having wavelengths that are greater than theabsorption edge 453 (or having a photon energy that is less than theband gap energy 441) travel in the semiconductor 122 and may excitecharges trapped in the deep-level traps formed by defects, such as theelectron 446 in the trap 444 of FIG. 4A. The photons in the light beam114 transfer energy to the trapped charges, and the transferred energymay be sufficient to migrate trapped charges into the conduction band442, where the charges flow as current. For example, these photons mayincrease the thermal energy of the trapped charges such that the chargesare able to migrate to the conduction band 442. Additionally oralternatively, such photons may provide energy to charges that haveaccumulated in the valence band 443 to migrate these charges to the trap444. Other photons from the beam 114 may further excite these charges tocause the charges to migrate from the trap 444 to the conduction band442 to flow as electrical current.

Thus, illuminating the semiconductor 122 with the light beam 114releases trapped charges (such as the electron 446 of FIG. 4A) andexcites these charges to the conduction band 442. Illuminating thesemiconductor 122 with the light beam 114 also may increase the amountof charge in the valence band 443 that migrates to the conduction band442. Once in the conduction band 442, the charges flow as electricalcurrent, thereby increasing the conductivity and decreasing theresistivity of the semiconductor 122. When the conductivity isincreased, the DLO 330 decreases. Moreover, removing or reducing thetrapped charges results in a more uniform electric field within thesemiconductor 122 when the potential difference V is applied. Further,illuminating the semiconductor 122 with the seed light beam 114 also mayreduce the acoustic leakage 329 and also may modify the nature of theacoustic effect in the semiconductor 122. For example, one or more ofthe frequency and amplitude of the acoustic effect (or acoustic leakage)may change. As such, by illuminating the semiconductor 122 with the seedlight beam 114, the optical leakage is reduced and the amount of lightin the pedestal portions 325 a, 325 b is reduced.

FIGS. 3A and 3B show an example in which the light beam 106 is acontinuous wave light beam. However, in some implementations, the lightbeam 106 is a pulsed light beam that includes a train of pulses of lightseparated in time, and synchronous to the voltage potential V appliedacross the crystal. The modulation module 120 also may be used to formoptical pulses from any of the pulses in the pulsed light beam. FIGS. 5Aand 5B relate to an optical pulse 507 formed from a pulse of light.

In implementations in which the light beam 106 is a pulsed light beam,the control system 175 (FIGS. 1 and 2A) may provide a trigger to thelight source 105 such that a pulse of light is emitted while thepotential difference is applied to the semiconductor 122. For example,the light source 105 may be a Q-switched CO₂ laser that is controlled toemit a pulse by receiving a trigger from the control system 175. In someimplementations, the light source 105 emits pulses on a periodic basiswithout receiving a trigger from the control system 175.

FIG. 5A is a plot of light transmitted by the optical modulator 120 as afunction of time. The vertical axis of FIG. 5A is a logarithmic scale.FIG. 5B is a plot that shows voltage applied to the semiconductor 122during the time shown in FIG. 5A. The optical pulse 507 is formed byapplying the potential difference V to the semiconductor 122 while allor a portion of the pulse of the light beam 106 propagates in thesemiconductor 122. Applying the potential difference to thesemiconductor 122 while the pulse propagates in the semiconductor 122allows the polarization of the pulse to be changed such that thepolarization-based optical element 124 (FIG. 2A) directs the light thatemerges from the semiconductor 122 onto the beam path 111 (FIG. 2A).

In the example of FIGS. 5A and 5B, the potential difference V is appliedto the semiconductor 122 during a period 537 that begins at time t=t8and ends at time t=t9. The potential difference V is not applied at theother times shown on FIG. 5B. When the potential difference V is appliedat time t=t8, the index of refraction of the semiconductor 122 changesinstantaneously, such that the polarization of light passing through thesemiconductor 122 matches the polarization-based optical element 124 andexits the optical modulator 120. Between the times t7 and t8, light fromthe pulse of the light beam 106 passes through the modulation module 120due to optical leakage, forming a pedestal portion 525. The opticalleakage may include DLO 330 and/or acoustic leakage 329 caused by anacoustic wave that formed in the semiconductor during a priorapplication of the potential difference V. By illuminating thesemiconductor 122 with the seed light beam 114, the maximum amount oflight or the average amount of light in the pedestal 525 may be changed,reduced, or eliminated. In the example of FIG. 5A, a window 521 showsthe portion of the pulse 507 that is the pedestal 525.

Referring to FIG. 6, a flow chart for a process 600 is shown. Theprocess 600 is an example of a process for producing an optical pulse,such as the optical pulse 107 (FIGS. 1 and 2B), for use in the EUV lightsource 101 of FIG. 1 or any other EUV light source. The process 600 maybe used to control or modify one or more characteristics of a pedestalportion of an optical pulse. For example, the process 600 may be used toreduce an average or maximum intensity of the pedestal portion of theproduced optical pulse. The process 600 is discussed with respect to theEUV light source 101 of FIG. 1 and the modulation module 120 (FIGS. 1and 2A).

The semiconductor 122 of the modulation module 120 is illuminated with afirst light beam having a first wavelength (610). The first light beammay be the seed light beam 114. The first wavelength is any wavelengththat is capable of liberating electrical charges trapped in thesemiconductor 122. Defects in the semiconductor 122 may act as siteswhere electrical charges are trapped, such as the deep-level trap 444(FIG. 4A). The trapped charges result in a lower conductivity and higherresistivity compared to an ideal version of the semiconductor 122 thatlacks defects and trapped charges. The traps formed by the defects haveband gap energies that are less than the band gap energy 441 of thesemiconductor 122. Thus, trapped charges may be liberated from thedefect traps by exciting the charges with light having a wavelengthassociated with a photon energy that is less than the band gap energy441.

For example, the semiconductor 122 may be CZT, which has a band gapenergy corresponding to a wavelength of 730 nm, or CdTe, which has aband gap energy corresponding to a wavelength of 861 nm. In theseexamples, the seed light source 110 may be any light source thatproduces near-infrared (NIR) light that includes light of a wavelengthwith a photon energy less than the band gap energy of the semiconductor122. NIR light includes wavelengths between 750 nm and 3.5 microns (μm).For example, a diode laser that has a peak wavelength of 905.7 nm and afull-width at half max (FWHM) of 1 nm or a light emitting diode that hasa peak wavelength of 873 nm and a FWHM of 70 nm may be used as the seedlight source 110. In another example, the seed light beam 114 mayinclude one or more wavelengths between 838 nm and 988 nm. The firstwavelength may be associated with a photon energy that is greater thanthe trap gap energy 447. As discussed above, the trap gap energy 447 isless than the band gap energy 441.

A voltage is applied to the semiconductor 122 for a time duration (620).An optical pulse (such as the optical pulse 107) is formed by passing asecond light beam having a second wavelength through the semiconductorduring the time duration (630). The second light beam may be, forexample, the light beam 106 of FIG. 1. The light beam 106 may becontinuous wave or a beam that includes one or more pulses of light.

The second light beam includes at least one wavelength that has a photonenergy far below the band gap 441 of the semiconductor 122. Thus, thesecond light beam is transmitted through the semiconductor and is usedto form the pulse 107. The light beam 106 includes one or morewavelengths that are different from the first wavelength. The light beam106 may include a wavelength that is no more than 3.5 times greater(longer) than the peak wavelength of the first light beam. In someimplementations, the light beam 106 includes a wavelength that is nomore than ten times greater (longer) than the peak wavelength of theseed light beam 114. Continuing the above example, the semiconductor 122may be CZT, which has a band gap of 730 nm, or CdTe, which has a bandgap of 861 nm. In these examples, the second light beam may be producedby a CO₂ laser and may include one or more wavelengths between 8 μm and15 μm or one or more wavelengths between 9 μm and 11 μm. The light beam106 and the light beam 114 may propagate in the semiconductor 122 at thesame time. Additionally, the light beam 106 and the light beam 114 mayfollow the same path through the semiconductor 122. It may be possiblefor the effects of irradiating the semiconductor 122 with the seed lightbeam 114 to persist after the seed light beam 114 is no longerilluminating the semiconductor 122. For example, the electricalproperties (such as the leakage current and resistivity) may become morestable (with less variation in time) after illumination with the seedlight beam 114.

Illuminating the semiconductor 122 with the seed light beam 114liberates trapped charges in the semiconductor 122, increasing theconductivity of the semiconductor 122 and decreasing the optical leakageof the modulation module 120. The decrease in optical leakage alsodecreases the amount of light in the pedestal portion of the pulse 107.FIGS. 7-10 show experimental results obtained by illuminating thesemiconductor 122 with the seed light beam 114.

FIG. 7 includes current-voltage (I-V) plots 701 and 702 that showmeasured current (microAmps) versus voltage (kiloVolts) applied to thesemiconductor 122 without irradiation by the seed light beam 114 (I-Vplot 701) and with irradiation by the seed light beam 114 (I-V plot702). In the example of FIG. 7, CZT was used as the semiconductor 122.The applied voltage was varied with a direct current (DC) power supplyfrom 0 V to 5000 V (5 kV). A 2 megaohm (MΩ) sampling resistor wasconnected to the semiconductor 122, and the leakage current was measuredwith a high-voltage probe across the 2 MΩ resistor. The leakage currentwas measured first without illuminating the semiconductor 122, and thesemeasured currents are included in the plot 701. A NIR source was used asthe seed light source 110. The leakage current was measured while thesemiconductor 122 was being illuminated with the seed light beam 114,and these measured leakage currents are included in the plot 702.

As seen in FIG. 7, illuminating the semiconductor 122 with the seedlight beam 114 increased the leakage current at all applied voltagesgreater than zero. The increased leakage current indicates that trappedcharges were liberated due to the irradiation by the seed light beam114. This increase in leakage current due to illumination with the seedlaser beam can also be seen as an increase in the photoconductivity ofthe semiconductor.

FIG. 8 includes a plot 800 of total optical leakage (%) as a function ofresistivity (at 1 kV-DC in Ohm centimeters) of the semiconductor 122. Inthe plot 800, the semiconductor 122 was CZT. The seed light beam 114illuminated the semiconductor 122 while the data was collected. The seedlight beam 114 was a NIR beam and the intensity of the beam wasincreased during the data collection from zero to a maximum of 1microWatt (μW) coupled into the semiconductor. As shown, the totaloptical leakage decreases as the resistivity of the semiconductor 122decreases and the intensity of the seed light beam 114 increases.Resistivity is inversely proportional to conductivity, thus, the plot800 indicates that the optical leakage decreases with increasedconductivity and increased intensity of the seed light beam 114. Theincreased conductivity corresponds to liberation of trapped chargesthrough excitation with the seed light beam 114. The plot 800 alsoindicates that different optical leakages may be obtained by varying theintensity of the seed light beam 114. As such, in addition to reducingthe amount of optical leakage by varying the intensity of the seed lightbeam 114, the amount of optical leakage (and thus control the amount oflight in the pedestal of the pulse 107) may be controlled or varied.

FIG. 9 shows experimentally measured optical leakage as a function oftime. The plot 901 shows the optical leakage when the semiconductor 122is not illuminated with the seed light beam 114, and the plot 902 showsthe optical leakage when the semiconductor 122 is illuminated with theseed light beam 114. In the example of FIG. 9, the light beam 106 was acontinuous wave light beam generated by a CO₂ laser. The seed light beam114 was a continuous wave NIR light beam. The pulses 907_1 and 907_2were formed using the modulation module 120.

As shown by comparing the plot 901 and the plot 902 of FIG. 9,illuminating the semiconductor 122 with the seed light beam 114 reducesthe optical leakage. The maximum optical leakage within a pedestalregion 921 is reduced from about 1.1% (without the seed light beam 114)to about 0.05% (with the seed light beam) in this particular example.Additionally, the optical leakage that occurs between the pulses 907_1and 907_2 (including components of the leakage attributable to theacoustic leakage) is also reduced from a maximum of about 1.4% (withoutthe seed light beam 114) to a maximum of about 0.3% (with the seed lightbeam 114). Considering the acoustic leakage alone without the DLO, thehighest peak-to-valley was about 0.6% without illumination and is about0.2% with illumination. Thus, in this example, there was about athree-fold reduction in the acoustic leakage due to illuminating thesemiconductor 122 with the seed light beam. In other examples,additional reduction of the acoustic leakage (for example, a six-foldreduction) may be achieved. Illuminating the semiconductor 122 with theseed light beam 114 does not eliminate or substantially change thestatic leakage, which is shown as a trace 927 in FIG. 9.

FIG. 10 shows pulses 1007_a and 1007_b as measured sensor voltage versustime. The pulse 1007_a was formed with the optical modulator 120 withoutilluminating the semiconductor 122 with the seed light beam 114. Thepulse 1007_b was formed with the optical modulator 120 while thesemiconductor 122 was illuminated with the seed light beam 114. In theexample of FIG. 10, the light beam 106 was a pulsed light beam producedby a CO₂ laser. The pulses 1007_a and 1007 b were each formed from aseparate pulse in the pulsed light beam, but are shown on the same timescale for comparison purposes. The light passing through the modulationmodule 120 was measured with a sensor that produces voltage in responseto detecting light. The voltage produced by the sensor was measured overtime and used to produce the plots shown in FIG. 10.

The pulses 1007_a and 1007_b have respective pedestal portions 1025 a,1025 b that occur over a time window 1021. The maximum voltage measuredby the sensor for the pedestal 1025 a was about 0.8 millivolts (mV) inthis example. The maximum voltage measured by the sensor for thepedestal 1025 b was less than 0.1 mV. Thus, illuminating thesemiconductor 122 with the seed light beam 114 reduced the amount oflight in the pedestal. The maximum voltage measured by the sensor for apedestal portion of pulse produced while the semiconductor 122 was beingilluminated may be 10-20 times lower compared to a pedestal portion of apulse produced while the semiconductor 122 was not being illuminatedwith the seed light beam. For example, for the first 1-2 hours of use ofa system that uses the modulation module 120, the semiconductor 122 isheated from an initial unheated state by the light beam 106 and theapplication of the potential difference V. The reduction in the pedestalportion may be about 20 times when the semiconductor 122 is in theunheated state. As the semiconductor 122 warms from the light beam 106and the application of the potential difference V, the reduction in thepedestal portion may be around 10 times.

The data shown in FIGS. 8-10 were measured with the same voltage sensor,a PEM sensor available from Boston Electronics, however, the data wasnormalized to the peak intensity of the produced pulse.

FIGS. 11 and 12 show perspective views of pulse generating systems 1104(FIG. 11) and 1204 (FIG. 12). The pulse generating systems 1104 and 1204are examples of implementations of the pulse generating system 104 andmay be used in the EUV light source 101 (FIG. 1).

In the pulse generating system 1104, the light beam 106 and the seedlight beam 114 are combined by a dichroic optical element 1161. Thedichroic optical element 1161 may be any optical element capable ofinteracting with light based on the wavelength of the light. Forexample, the dichroic optical element 1161 may be a dichroic mirror thattransmits the first wave length (the wavelength of the light beam 106)and reflects the second wavelength (the wavelength of the seed lightbeam 114).

After interacting with the dichroic optical element 1161, the light beam106 and the seed light beam 114 interact with the semiconductor 122(FIGS. 1 and 2A) of the optical modulator 120. The light beam 106 andthe seed light beam 114 follow the same path through the semiconductor122. Additionally, the light beam 106 and the seed light beam 114 maypropagate in the semiconductor 122 at the same time. A portion of thelight beam 106 is extracted by applying the potential difference V tothe semiconductor 122, and the pulse 107 is formed. A portion of theseed light beam 114 also may pass through the optical modulator 120.

In the pulse generating system 1204, the light beam 106 is incident onthe semiconductor 122 at a side 1262 and the seed light beam 114 isincident on the semiconductor 122 at a side 1263. The sides 1262 and1263 are different sides, and the seed light beam 114 and the light beam106 propagate in different directions in the semiconductor 122. In theimplementation of FIG. 12, the seed light beam 114 is diffused ordiverged such that the seed light beam 114 illuminates more than oneportion of the semiconductor at the same time. The seed light beam 114may be diffused with a diffuser element (not shown) such as apolytetrafluoroethylene (PTFE) diffuser. The diffuser element is placedbetween the seed light source 110 and the semiconductor 122.

Referring to FIG. 13A, an LPP EUV light source 1300 is shown. The pulsegenerating systems 104, 1104, and 1204 may be part of an EUV lightsource, such as the source 1300. The pulse generating systems 104, 1104,and 1204 are not shown in FIG. 13A. However, the pulse generatingsystems 104, 1104, and 1204 may be positioned in the light source 1300as part of the beam transport system 1320, for example. In theseimplementations, the light source 105 of the systems 104, 1104, and 1204is part of the drive laser 1315, and the control system 175 may be partof the master controller 1355, any of the components of the mastercontroller 1355, or may be implemented as a separate control system.

The LPP EUV light source 1300 is formed by irradiating a target mixture1314 at a target region 1305 with an amplified light beam 1310 thattravels along a beam path toward the target mixture 1314. The targetmaterial of the target 118 may be or include the target mixture 1314.The target region 1305 is within an interior 1307 of a vacuum chamber1330. When the amplified light beam 1310 strikes the target mixture1314, a target material within the target mixture 1314 is converted intoa plasma state that has an element with an emission line in the EUVrange. The created plasma has certain characteristics that depend on thecomposition of the target material within the target mixture 1314. Thesecharacteristics may include the wavelength of the EUV light produced bythe plasma and the type and amount of debris released from the plasma.

The light source 1300 also includes a target material delivery system1325 that delivers, controls, and directs the target mixture 1314 in theform of liquid droplets, a liquid stream, solid particles or clusters,solid particles contained within liquid droplets or solid particlescontained within a liquid stream. The target mixture 1314 includes thetarget material such as, for example, water, tin, lithium, xenon, or anymaterial that, when converted to a plasma state, has an emission line inthe EUV range. For example, the element tin may be used as pure tin(Sn); as a tin compound, for example, SnBr₄, SnBr₂, SnH₄; as a tinalloy, for example, tin-gallium alloys, tin-indium alloys,tin-indium-gallium alloys, or any combination of these alloys. Thetarget mixture 1314 may also include impurities such as non-targetparticles. Thus, in the situation in which there are no impurities, thetarget mixture 1314 is made up of only the target material. The targetmixture 1314 is delivered by the target material delivery system 1325into the interior 1307 of the chamber 1330 and to the target region1305.

The light source 1300 includes a drive laser system 1315 that producesthe amplified light beam 1310 due to a population inversion within thegain medium or mediums of the laser system 1315. The light source 1300includes a beam delivery system between the laser system 1315 and thetarget region 1305, the beam delivery system including a beam transportsystem 1320 and a focus assembly 1322. The beam transport system 1320receives the amplified light beam 1310 from the laser system 1315, andsteers and modifies the amplified light beam 1310 as needed and outputsthe amplified light beam 1310 to the focus assembly 1322. The focusassembly 1322 receives the amplified light beam 1310 and focuses thebeam 1310 to the target region 1305.

In some implementations, the laser system 1315 may include one or moreoptical amplifiers, lasers, and/or lamps for providing one or more mainpulses and, in some cases, one or more pre-pulses. Each opticalamplifier includes a gain medium capable of optically amplifying thedesired wavelength at a high gain, an excitation source, and internaloptics. The optical amplifier may or may not have laser mirrors or otherfeedback devices that form a laser cavity. Thus, the laser system 1315produces an amplified light beam 1310 due to the population inversion inthe gain media of the laser amplifiers even if there is no laser cavity.Moreover, the laser system 1315 may produce an amplified light beam 1310that is a coherent laser beam if there is a laser cavity to provideenough feedback to the laser system 1315. The term “amplified lightbeam” encompasses one or more of: light from the laser system 1315 thatis merely amplified but not necessarily a coherent laser oscillation andlight from the laser system 1315 that is amplified and is also acoherent laser oscillation.

The optical amplifiers in the laser system 1315 may include as a gainmedium a filling gas that includes CO₂ and may amplify light at awavelength of between about 9100 and about 11000 nm, and in particular,at about 10600 nm, at a gain greater than or equal to 800 times.Suitable amplifiers and lasers for use in the laser system 1315 mayinclude a pulsed laser device, for example, a pulsed, gas-discharge CO₂laser device producing radiation at about 9300 nm or about 10600 nm, forexample, with DC or RF excitation, operating at relatively high power,for example, 10 kW or higher and high pulse repetition rate, forexample, 40 kHz or more. The pulse repetition rate may be, for example,50 kHz. The optical amplifiers in the laser system 1315 may also includea cooling system such as water that may be used when operating the lasersystem 1315 at higher powers.

FIG. 13B shows a block diagram of a drive laser system 1380. The drivelaser system 1380 may be used as part of the drive laser system 1315 inthe source 1300. The drive laser system 1380 includes three (or more)power amplifiers 1381, 1382, and 1383. Any or all of the poweramplifiers 1381, 1382, and 1383 may include internal optical elements(not shown).

Light 1384 exits the power amplifier 1381 through an output window 1385and is reflected off a curved mirror 1386. After reflection, the light1384 passes through a spatial filter 1387, is reflected off of a curvedmirror 1388, and enters the power amplifier 1382 through an input window1389. The light 1384 is amplified in the power amplifier 1382 andredirected out of the power amplifier 1382 through an output window 1390as light 1391. The light 1391 is directed toward the amplifier 1383 witha fold mirror 1392 and enters the amplifier 1383 through an input window1393. The amplifier 1383 amplifies the light 1391 and directs the light1391 out of the amplifier 1383 through an output window 1394 as anoutput beam 1395. A fold mirror 1396 directs the output beam 1395 upward(out of the page) and toward the beam transport system 1320 (FIG. 13A).

Referring again to FIG. 13B, the spatial filter 1387 defines an aperture1397, which may be, for example, a circle having a diameter betweenabout 2.2 mm and 3 mm. The curved mirrors 1386 and 1388 may be, forexample, off-axis parabola mirrors with focal lengths of about 1.7 m and2.3 m, respectively. The spatial filter 1387 may be positioned such thatthe aperture 1397 coincides with a focal point of the drive laser system1380.

Referring again to FIG. 13A, the light source 1300 includes a collectormirror 1335 having an aperture 1340 to allow the amplified light beam1310 to pass through and reach the target region 1305. The collectormirror 1335 may be, for example, an ellipsoidal mirror that has aprimary focus at the target region 1305 and a secondary focus at anintermediate location 1345 (also called an intermediate focus) where theEUV light may be output from the light source 1300 and may be input to,for example, an integrated circuit lithography tool (not shown). Thelight source 1300 may also include an open-ended, hollow conical shroud1350 (for example, a gas cone) that tapers toward the target region 1305from the collector mirror 1335 to reduce the amount of plasma-generateddebris that enters the focus assembly 1322 and/or the beam transportsystem 1320 while allowing the amplified light beam 1310 to reach thetarget region 1305. For this purpose, a gas flow may be provided in theshroud that is directed toward the target region 1305.

The light source 1300 may also include a master controller 1355 that isconnected to a droplet position detection feedback system 1356, a lasercontrol system 1357, and a beam control system 1358. The light source1300 may include one or more target or droplet imagers 1360 that providean output indicative of the position of a droplet, for example, relativeto the target region 1305 and provide this output to the dropletposition detection feedback system 1356, which may, for example, computea droplet position and trajectory from which a droplet position errormay be computed either on a droplet by droplet basis or on average. Thedroplet position detection feedback system 1356 thus provides thedroplet position error as an input to the master controller 1355. Themaster controller 1355 may therefore provide a laser position,direction, and timing correction signal, for example, to the lasercontrol system 1357 that may be used, for example, to control the lasertiming circuit and/or to the beam control system 1358 to control anamplified light beam position and shaping of the beam transport system1320 to change the location and/or focal power of the beam focal spotwithin the chamber 1330.

The target material delivery system 1325 includes a target materialdelivery control system 1326 that is operable, in response to a signalfrom the master controller 1355, for example, to modify the releasepoint of the droplets as released by a target material supply apparatus1327 to correct for errors in the droplets arriving at the desiredtarget region 1305.

Additionally, the light source 1300 may include light source detectors1365 and 1370 that measures one or more EUV light parameters, includingbut not limited to, pulse energy, energy distribution as a function ofwavelength, energy within a particular band of wavelengths, energyoutside of a particular band of wavelengths, and angular distribution ofEUV intensity and/or average power. The light source detector 1365generates a feedback signal for use by the master controller 1355. Thefeedback signal may be, for example, indicative of the errors inparameters such as the timing and focus of the laser pulses to properlyintercept the droplets in the right place and time for effective andefficient EUV light production.

The light source 1300 may also include a guide laser 1375 that may beused to align various sections of the light source 1300 or to assist insteering the amplified light beam 1310 to the target region 1305. Inconnection with the guide laser 1375, the light source 1300 includes ametrology system 1324 that is placed within the focus assembly 1322 tosample a portion of light from the guide laser 1375 and the amplifiedlight beam 1310. In other implementations, the metrology system 1324 isplaced within the beam transport system 1320. The metrology system 1324may include an optical element that samples or re-directs a subset ofthe light, such optical element being made out of any material that maywithstand the powers of the guide laser beam and the amplified lightbeam 1310. A beam analysis system is formed from the metrology system1324 and the master controller 1355 since the master controller 1355analyzes the sampled light from the guide laser 1375 and uses thisinformation to adjust components within the focus assembly 1322 throughthe beam control system 1358.

Thus, in summary, the light source 1300 produces an amplified light beam1310 that is directed along the beam path to irradiate the targetmixture 1314 at the target region 1305 to convert the target materialwithin the mixture 1314 into plasma that emits light in the EUV range.The amplified light beam 1310 operates at a particular wavelength (thatis also referred to as a drive laser wavelength) that is determinedbased on the design and properties of the laser system 1315.Additionally, the amplified light beam 1310 may be a laser beam when thetarget material provides enough feedback back into the laser system 1315to produce coherent laser light or if the drive laser system 1315includes suitable optical feedback to form a laser cavity.

Other implementations are within the scope of the claims.

What is claimed is:
 1. A method of forming an optical pulse for anextreme ultraviolet (EUV) light source, the method comprising:illuminating a semiconductor material of a modulation system with afirst light beam having a first wavelength; applying a voltage to thesemiconductor material for a time duration, the applied voltage beingsufficient to modify an index of refraction of the semiconductormaterial such that a polarization state of a light beam having a secondwavelength passing through the semiconductor material is modified topass through at least one polarization-based optical element of themodulation system; forming an optical pulse by passing a second lightbeam having the second wavelength through the semiconductor materialduring the time duration, wherein: the formed optical pulse comprises afirst portion and a second portion, the first portion and the secondportion being temporally contiguous, the first portion occurring beforethe second portion; and adjusting a property of the first light beam toadjust one or more characteristics of the first portion of the formedoptical pulse, wherein illuminating the semiconductor material of themodulation system with the first light beam modifies one or morecharacteristics of the first portion of the formed optical pulse,wherein the second portion of the formed optical pulses has an intensitysufficient to convert target material in a target at a target region toa plasma that emits EUV light.
 2. The method of claim 1, wherein the oneor more modified characteristics of the first portion of the formedoptical pulse comprise an average intensity, a maximum intensity, and/ora temporal duration.
 3. The method of claim 1, further comprisingallowing the formed pulse to propagate toward the target region, thefirst portion of the formed pulse having a maximum intensity that isless than a maximum intensity of the second portion of the formed pulse.4. The method of claim 1, wherein: the semiconductor material isassociated with a spectral transmission characteristic, the spectraltransmission characteristic comprising a transmission region and anabsorption edge wavelength, the absorption edge wavelength being thelowest wavelength on the transmission region, the first wavelength isbetween the absorption edge wavelength and the second wavelength, andthe second wavelength is a wavelength in the transmission region.
 5. Themethod of claim 1, wherein the semiconductor material is associated witha band gap energy, the band gap energy being an energy differencebetween a valence band of the semiconductor material and a conductionband of the semiconductor material, and the photon energy of the firstwavelength is less than the band gap energy.
 6. The method of claim 5,wherein the semiconductor material comprises defects, the defectscreating deep-level traps with energy levels between the valence bandand the conduction band, and the photon energy of the first wavelengthis equal to or greater than an energy difference between at least oneenergy level of a deep-level trap and the conduction band or between atleast one energy level of a deep-level trap and the valence band.
 7. Themethod of claim 1, wherein the second wavelength comprises 10.6 μm, andthe semiconductor material comprises one of cadmium zinc telluride(CdZnTe), cadmium telluride (CdTe), zinc telluride (ZnTe), and galliumarsenide (GaAs).
 8. The method of claim 1, wherein the first wavelengthcomprises a wavelength between 0.75 microns (μm) and 3.5 μm, and thesecond wavelength comprises a wavelength between 9 μm and 11 μm.
 9. Themethod of claim 1, wherein the first light beam and the second lightbeam follow the same spatial path through the semiconductor material.10. The method of claim 9, wherein the first light beam and the secondlight beam are in the semiconductor material at the same time.
 11. Themethod of claim 1, wherein adjusting the property of the first lightbeam comprises increasing an intensity of the first light beam to reducea maximum or average intensity of the first portion of the opticalpulse.
 12. The method of claim 1, wherein modifying one or morecharacteristics of the first portion of the formed optical pulsecomprises reducing a maximum and/or average intensity of the firstportion of the optical pulse.
 13. An apparatus comprising: a modulationsystem comprising a semiconductor material; and a control systemconfigured to: illuminate the semiconductor material of the modulationsystem with a first light beam having a first wavelength; apply avoltage to the semiconductor material for a time duration, the appliedvoltage being sufficient to modify an index of refraction of thesemiconductor material such that a polarization state of a light beamhaving a second wavelength passing through the semiconductor material ismodified to pass through at least one polarization-based optical elementof the modulation system, form an optical pulse by passing a secondlight beam having the second wavelength through the semiconductormaterial during the time duration, wherein the formed optical pulsecomprises a first portion and a second portion, the first portion andthe second portion being temporally contiguous, the first portionoccurring before the second portion, and adjust a property of the firstlight beam to adjust one or more characteristics of the first portion ofthe optical pulse, wherein illuminating the semiconductor material ofthe modulation system with the first light beam modifies one or morecharacteristics of the first portion of the formed optical pulse,wherein the apparatus is configured for use in an EUV light source, andthe formed optical pulse has an energy sufficient to convert at leastsome target material to plasma that emits EUV light.
 14. The apparatusof claim 13, further comprising: a first light source coupled to thecontrol system, the first light source configured to produce the firstlight beam having the first wavelength; and a second light sourcecoupled to the control system, the second light source configured toproduce the second light beam having the second wavelength.
 15. Theapparatus of claim 14, wherein the second light source comprises apulsed light source, the control system is coupled to the modulationsystem and the second light source, and the control system is configuredto control the second light source to emit a pulse of light.
 16. Theapparatus of claim 15, wherein at least one pulse of first light beamand the second light beam are in the semiconductor material at the sametime.
 17. The apparatus of claim 14, wherein the control system isfurther configured to control the first light source to control one ormore characteristics of the first portion of the optical pulse.
 18. Theapparatus of claim 17, wherein the one or more characteristics of thefirst portion comprise one or more of an average intensity, a maximumintensity, and a temporal duration.
 19. The apparatus of claim 18,wherein the control system is configured to control an intensity of thefirst portion of the pulse by controlling the first light source. 20.The apparatus of claim 13, wherein the second light beam comprises acontinuous light beam.
 21. An EUV light source comprising: a lightgeneration module configured to produce a pulsed light beam having afirst wavelength; a modulation system positioned to interact with thepulsed light beam, the modulation system comprising a semiconductormaterial, wherein the semiconductor material is associated with aspectral transmission characteristic, the spectral transmissioncharacteristic comprising a transmission region and an absorption edgewavelength, the absorption edge wavelength being the lowest wavelengthon the transmission region, the first wavelength being in thetransmission region; a control system coupled to the modulation system,the control system configured to: illuminate the semiconductor materialof the modulation system with a seed light beam having a seed wavelengthbetween the absorption edge wavelength and the first wavelength; apply avoltage to the semiconductor material for a time duration, the appliedvoltage being sufficient to modify an index of refraction of thesemiconductor material such that a polarization state of a light beamhaving the first wavelength passing through the semiconductor materialis modified to pass through at least one polarization-based opticalelement of the modulation system, and form an optical pulse by passingat least one pulse of the pulsed light beam through the semiconductormaterial during the time duration, wherein the formed optical pulsecomprises a first portion and a second portion, the first portion andthe second portion being temporally contiguous, the first portionoccurring before the second portion, and illuminating the semiconductormaterial of the modulation system with the first light beam modifies oneor more characteristics of the first portion of the formed opticalpulse; a vacuum vessel configured to receive a target material and theformed optical pulse, an interaction between the target material and theformed optical pulse producing a plasma that emits EUV light; and atleast one optical element in the vacuum vessel positioned to direct theEUV light toward an optical lithography system.
 22. An apparatuscomprising: a modulation system comprising a semiconductor material,wherein the semiconductor material is associated with a spectraltransmission characteristic, the spectral transmission characteristiccomprising a transmission region and an absorption edge wavelength, theabsorption edge wavelength being the lowest wavelength on thetransmission region, a first wavelength being in the transmissionregion; and a control system configured to: illuminate the semiconductormaterial of the modulation system with a first light beam having thefirst wavelength, the first wavelength being between the absorption edgewavelength and a second wavelength; apply a voltage to the semiconductormaterial for a time duration, the applied voltage being sufficient tomodify an index of refraction of the semiconductor material such that apolarization state of a light beam having the second wavelength in thetransmission region of the semiconductor material that passes throughthe semiconductor material is modified to pass through at least onepolarization-based optical element of the modulation system, and form anoptical pulse by passing a second light beam having the secondwavelength through the semiconductor material during the time duration,wherein the formed optical pulse comprises a first portion and a secondportion, the first portion and the second portion are temporallycontiguous, the first portion occurs before the second portion, andilluminating the semiconductor material of the modulation system withthe first light beam modifies one or more characteristics of the firstportion of the formed optical pulse, wherein the apparatus is configuredfor use in an EUV light source, and the formed optical pulse has anenergy sufficient to convert at least some target material to plasmathat emits EUV light.
 23. The apparatus of claim 22, whereinilluminating the semiconductor material of the modulation system withthe first light beam modifies a maximum intensity, an average intensity,and/or a temporal duration of the first portion of the formed opticalpulse.
 24. The apparatus of claim 22, wherein illuminating thesemiconductor material of the modulation system with the first lightbeam reduces a maximum intensity and/or an average intensity of thefirst portion of the formed optical pulse.
 25. An apparatus comprising:a modulation system comprising a semiconductor material, wherein thesemiconductor material is associated with a band gap energy, the bandgap energy being an energy difference between a valence band of thesemiconductor material and a conduction band of the semiconductormaterial; and a control system configured to: illuminate thesemiconductor material of the modulation system with a first light beamhaving a first wavelength, wherein a photon energy of the firstwavelength is less than the band gap energy; apply a voltage to thesemiconductor material for a time duration, the applied voltage beingsufficient to modify an index of refraction of the semiconductormaterial such that a polarization state of a light beam having a secondwavelength in a transmission region of the semiconductor material thatpasses through the semiconductor material is modified to pass through atleast one polarization-based optical element of the modulation system,and form an optical pulse by passing a second light beam having thesecond wavelength through the semiconductor material during the timeduration, wherein the formed optical pulse comprises a first portion anda second portion, the first portion and the second portion aretemporally contiguous, the first portion occurs before the secondportion, and illuminating the semiconductor material of the modulationsystem with the first light beam modifies one or more characteristics ofthe first portion of the formed optical pulse, wherein the apparatus isconfigured for use in an EUV light source, and the formed optical pulsehas an energy sufficient to convert at least some target material toplasma that emits EUV light.
 26. The apparatus of claim 25, whereinilluminating the semiconductor material of the modulation system withthe first light beam modifies a maximum intensity, an average intensity,and/or a temporal duration of the first portion of the formed opticalpulse.
 27. The apparatus of claim 25, wherein illuminating thesemiconductor material of the modulation system with the first lightbeam reduces a maximum intensity and/or an average intensity of thefirst portion of the formed optical pulse.